1. Field of the Invention
The present invention relates to a magneto-optical recording medium for reproducing an information signal while causing a magnetic domain wall movement by laser beam irradiation.
2. Description of the Related Art
A basic principle of a magneto-optical recording system is that a temperature of a ferrimagnetic thin film is locally raised to the vicinity of Curie point or a compensation point, and a coercive force of this portion is reduced to reverse a direction of magnetization toward an externally applied recording magnetic field for an information signal to be recorded. The magnetization reversal portion, i.e., an information bit, forms a magnetic domain and, in magneto-optical recording/reproducing for reading the information bit based on magnetic Kerr effect, in order to increase a recording density, it is necessary to shorten a recording bit length, that is, to micronize a magnetic domain when the information signal is recorded in the form of the magnetic domain. However, reproducing resolution of the magnetic domain (information signal) is practically decided by a wavelength λ of a laser beam source of an optical reproducing system and a numerical aperture NA of an objective lens, and a spatial frequency 2NA/λ is a reproducing limit. Accordingly, a conceivable way to increase a recording density is to shorten the wavelength λ of the laser beam source or to reduce a spot diameter of a laser beam in a reproducing device side by using an objective lens of high NA. However, a wavelength of a laser beam source currently at a practical level is only about 640 nm, and the use of the objective lens of high NA results in a shallow focal depth, which requires accuracy of a distance between the objective lens and a magneto-optical recording medium (optical disk or optical card). Consequently, manufacturing accuracy of the magneto-optical recording medium becomes difficult to obtain. Therefore, NA of the objective lens cannot be increased so high, and practical NA of the objective lens is only about 0.6. In other words, there is a limit to the increase of the recording density by the wavelength λ of the laser beam source and the numerical aperture NA of the objective lens.
In connection to this, to solve such problems of the recording density defined by conditions for reproducing, there is a signal reproducing method of a magnetic recording medium for executing reproducing on a magnetic recording medium (magneto-optical recording medium) where a magnetic layer is formed in a 3-layer structure (e.g., see Japanese Patent Laid-Open Hei No. 11(1999)-86372(p. 4, FIG. 1)). There is also a magnetic recording medium (magneto-optical recording medium) where a magnetic layer is formed in a 4-layer structure (e.g., see Japanese Patent Laid-Open No. 2000-187898 (p. 3 to 4, FIG. 1)).
FIG. 1 is a view for explaining an example of a conventional magneto-optical recording medium: (a) in FIG. 1 schematically shows a layer formation of the magneto-optical recording medium, and (b) in FIG. 1 shows a temperature distribution on the magneto-optical recording medium when the magneto-optical recording medium is irradiated with a laser beam. FIG. 2 is a view for explaining another example of a conventional magneto-optical recording medium: (a) in FIG. 2 schematically shows a layer formation of the magneto-optical recording medium, and (b) in FIG. 2 shows a temperature distribution on the magneto-optical recording medium when the magneto-optical recording medium is irradiated with a laser beam.
The conventional magneto-optical recording medium 110 in the example shown in FIG. 1 is disclosed in the above Japanese Patent Laid-Open No. Hei 11 (1999)-86372. The conventional magneto-optical recording medium 120 in the other example shown in FIG. 2 is disclosed in the above Japanese Patent Laid-Open No. 2000-187898. Here, description is briefly made by referring to the above publications.
First, as shown in FIG. 1, in the conventional magneto-optical recording medium 110 of the example, first to third magnetic layers 111 to 113 are sequentially laminated in a state of exchange coupling at room temperature. The first magnetic layer 111 disposed on a side irradiated with a reproducing laser beam to become a magnetic domain wall displacement layer is made of a magnetic film having a relatively small magnetic domain wall coercive force compared with the third magnetic layer 113 which becomes a recording layer. The second magnetic layer 112 is made of a magnetic layer having Curie temperature lower than those of the first and third magnetic layers 111 and 113.
More specifically, each of the above-described first to third magnetic layers 111 to 113 is made of a rare earth-iron family metal amorphous alloy containing 10 to 40 at. % of one or more types of rare earth metal elements such as Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er, and 60 to 90 at. % of one or more types of iron family elements such as Fe, Co, and Ni. It is also described that, to increase a corrosion resistance; a small amount of elements such as Cr, Mn, Cu, Ti, Al, Si, Pt, and In may be added to such an alloy.
In the magneto-optical recording medium 110 constituted in the foregoing manner, by laser beam irradiation from the first magnetic layer 111 side during recording, the information signal is recorded in the third magnetic layer 113 by a not-shown magnetic head to be saved as a magnetization reversal area of an arrow direction (hereinafter, described as a magnetic domain). Furthermore, after the recording, and during non-irradiation with a laser beam, the magnetic domain recorded in the third magnetic layer 113 is exchange-coupled to the first magnetic layer 111 through the second magnetic layer 112. In this case, vertical arrows AS in the first to third magnetic layers 111 to 113 indicate direction of atomic spins. On a boundary of areas where directions of spins are opposite to each other, a magnetic domain wall DW is formed.
Here, if the first magnetic layer 111 is irradiated with a laser beam during reproducing, a medium temperature reaches Curie temperature Ts or higher of the second magnetic layer 112 between shown positions X1 and X2 with respect to the laser beam. Correspondingly, in an area between the positions X1 and X2, because a temperature of the second magnetic layer 112 is raised to Curie temperature Ts or higher, magnetization thereof is lost to cut off the exchange coupling between the first and third magnetic layers 111 and 113. This area is referred to as a decoupling area.
Then, when the magnetic domain wall DW existing in the first magnetic layer 111 enters the decoupling area, this magnetic domain wall DW is moved toward a temperature peak in the first magnetic layer 111 as indicated by an arrow to generate magnetic domain wall movement DWM. Following this magnetic domain wall movement DWM, the magnetic domain exchange-coupled in the first magnetic layer 111 is enlarged by a reproducing laser beam to be read out. On the other hand, since a coercive force (magnetic domain wall coercive force) of the third magnetic layer 113 which becomes a recording layer is sufficiently large, the magnetic domain wall therein is not moved, and a recording state is maintained. Thus, the very small magnetic domain in which reproducing is impossible by normal reproducing resolution is enlarged to execute reproducing, whereby a recording density can be greatly increased.
Then, as shown in FIG. 2, in a conventional magneto-optical recording medium 120 of the other example, magnetic layers are formed by adding one more layer to the magneto-optical recording medium 110 of the foregoing example. Thus, performance is enhanced so that especially by laser beam irradiation, without simultaneous magnetic domain wall movements from the front and rear sides (directions of arrows I1 and I2) of a laser beam moving direction in a temperature elevated area which enables magnetic domain wall movement, only the magnetic domain wall movement from the front side (direction of the arrow I1) of the moving direction can be enlarged and read by a laser beam spot.
In the magneto-optical recording medium 120, first to fourth magnetic layers 121 to 124 are sequentially laminated in a state of exchange coupling at room temperature. The first magnetic layer 121 disposed on a side irradiated with a reproducing laser beam to become a magnetic domain wall displacement layer has a magnetic domain wall coercive force smaller than those of the second to fourth magnetic layers 122 to 124 at room temperature. The second magnetic layer 122 has a magnetic domain wall energy density higher than that of the first magnetic layer 121 at room temperature. The third magnetic layer 113 has Curie temperature set higher than room temperature, but lower than those of the first, second and fourth magnetic layers 121, 122, and 124. The fourth magnetic layer 124 is formed as a recording layer.
More specifically, each of the above-described first to fourth magnetic layers 121 to 124 is made, roughly similar to the first to third magnetic layers 111 to 113 of the magneto-optical recoding medium 110, of a rare earth-iron family metal amorphous alloy containing 10 to 40 at. % of one or more types of rare earth metal elements such as Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er, and 60 to 90 at. % of one or more types of iron family elements such as Fe, Co, and Ni. It is also described that, to increase a corrosion resistance, a small amount of elements such as Cr, Mn, Cu, Ti, Al, Si, Pt, and In may be added to such an alloy. Furthermore, it is described that a platinum family metal-iron family metal periodic structure film such as Pt/C, Pd/Co or the like, a platinum family metal-iron family metal alloy film, an antiferromagnetic material such as Co—Ni—O or Fe—Rh based alloy, and a material such as magnetic garnet can be used.
In the magneto-optical recording medium 120 constituted in the foregoing manner, by laser beam irradiation from the first magnetic layer 121 side during recording, an information signal is recorded in the fourth magnetic layer 124 by a not-shown magnetic head to be saved as a magnetic domain of an arrow direction. Furthermore, after the recording, and during non-irradiation with a laser beam, the magnetic domain recorded in the fourth magnetic layer 124 is exchange-coupled to the first magnetic layer 121 through the second and third magnetic layers 122 and 123. In this case, vertical arrows AS in the first to fourth magnetic layers 121 to 124 indicate directions of atomic spins. On a boundary of areas where directions of spins are opposite to each other, a magnetic domain wall DW is formed.
Here, if the first magnetic layer 121 is irradiated with a laser beam during reproducing, a medium temperature reaches Curie temperature Ts or higher of the third magnetic layer 123 between shown positions X1 and X2 with respect to the laser beam. Correspondingly, in an area between the positions X1 and X2, because a temperature of the third magnetic layer 123 is, raised to Curie temperature Ts or higher, magnetization thereof is lost to cut off the exchange coupling between the first and second magnetic layers 121 and 122, and the fourth magnetic layer 124. This area is referred to as a decoupling area.
Then, when the magnetic domain wall DW existing in the first and second magnetic layers 121 and 122 enters the decoupling area, this magnetic domain wall DW is moved toward a temperature peak in the first and second magnetic layers 121 and 122 as indicated by an arrow to generate magnetic domain wall movement DWM. Following this magnetic domain wall movement DWM, the magnetic domain exchange-coupled in the first magnetic layer 121 is enlarged by a reproducing laser beam to be read out. On the other hand, since a coercive force (magnetic domain wall coercive force) of the fourth magnetic layer 124 which becomes a recording layer is sufficiently large, the magnetic domain wall therein is not moved, and a recording state is maintained. Thus, the very small magnetic domain in which reproducing is impossible by normal reproducing resolution is enlarged to execute reproducing, whereby a recording density can be greatly increased.
In the Japanese Patent Laid-Open No. 2000-187898, it is described that the magnetic domain wall movement DWM occurs in the first and second magnetic layers 121 and 122. Generally, however, since the second magnetic layer 122 is formed very thin, whether the second magnetic layer 122 functions as a magnetic domain wall displacement layer or as a layer for controlling exchange coupling is not exactly known. Therefore, it may be considered that, during reproducing, a temperature is raised to lose at least magnetization of the third magnetic layer 123 by laser beam irradiation, and magnetic domain wall movement is generated so as to enlarge the magnetic domain exchange-coupled in the first magnetic layer 121.
Incidentally, in each of the conventional magneto-optical recording medial 110 and 120 respectively shown in FIG. 1 and FIG. 2, the magnetic domain enlargement reproducing technology by the magnetic domain wall movement DWM is effective for realizing a high-density recording medium, but there is a shift of a reproduced signal on a time axis, i.e., a problem of deteriorated jitter characteristics, which is intrinsic to the magnetic domain wall movement reproducing. To improve the jitter characteristics, a start timing of the magnetic domain wall movement DWM must be set sharp, and time until completion of the magnetic domain wall movement DWM must be shortened.
However, in each film composition of the first to third magnetic layers 111 to 113 and the first-to fourth magnetic layers 121 to 124 of the conventional magneto-optical recording media 110 and 120, as described above, each of all the magnetic layers only contains 10 to 40 at. % of one or more types of rare earth metal elements, and 90 to 60 at. % of one or more types of iron family elements such as Fe, Co, and Ni. Based on this, samples were prepared to try an experiment. However, this composition did not lead to any improvements of jitter characteristics during reproducing.